Microbially Influenced Corrosion
The mechanisms commonly thought to be involved in MIC include:
Microbes fall into two basic groups, aerobic and anaerobic. These two groups are
based on the kind of environment they prefer, either with or without oxygen. Slime
formers form a diverse group of aerobic bacteria. Common anaerobic bacteria include
Sulfur/sulfate reducing bacteria (SRB's) and organic acid formers.
- Cathodic depolarization, whereby the cathodic rate limiting step is accelerated
by micro-biological action.
- Formation of occluded surface cells, whereby microorganisms form "patchy" surface
colonies. Sticky polymers attract and aggregate biological and non-biological species
to produce crevices and concentration cells, the basis for accelerated attack.
- Fixing of anodic reaction sites, whereby microbiological surface colonies lead
to the formation of corrosion pits, driven by microbial activity and associated
with the location of these colonies.
- Underdeposit acid attack, whereby corrosive attack is accelerated by acidic final
products of the MIC "community metabolism", principally short-chain fatty acids.
Microbes tend to form colonies, with different characteristics from the outside
to inside. On the outside, "slimers" may produce polymers (slime) that attract inorganic
material, making the colony look like a pile of mud and debris. These aerobic organisms
can efficiently use up all available oxygen, giving anaerobic microbes (SRB's) inside
the colony a hospitable environment, allowing enhanced corrosion under the colony.
Microbially influenced corrosion (MIC) is a special danger when steels or alloys
of aluminum and copper are in constant contact with nearly neutral water, of pH
4 to 9, 50° to 122°F (10° to 50°C), especially when stagnant.
Microbially influenced corrosion mostly takes the form of pitting corrosion.
Corrosion products and effects include iron sulfates, slime, plugging, and bacteria
growths. Sulfate-reducing bacteria (SRB) are anaerobic bacteria which metabolize
sulfates (SO42-) and produce sulfuric acids or H2
S, thus introducing hydrogen sulfide into the system. SRB colonies can also form
deposits that are conducive to under-deposit corrosion (crevice corrosion.)
Water storage tanks are a common site where MIC occurs. SRB's can contaminate tanks,
which must then be cleaned and sterilized because it is impossible for biocides
to penetrate the large amounts of sludge and debris in tank bottoms. Flow lines
are another common MIC site, especially at the bottom of the line where water accumulates.
MIC has also been detected at the 3 o'clock and 9 o'clock positions, presumably
at the oil and water interface.
Prevention / Mitigation
To reduce or prevent microbial corrosion:
Drilling - biocides
Production - biocides, chlorine dioxide
Flowlines - biocides, chlorine dioxide
Cost considerations - Continuous vs. batch; EPA; biostat vs. biocide
Pictures - Click on thumbnail to see larger picture
Bacterial attack is usually characterized by rounded pits with etched sides, edges,
MIC pits often have a terraced effect.
Although MIC normally occurs at the bottom of the line where water accumulates,
it has also been detected at the 3 o'clock and 9 o'clock positions, presumably at
the oil and water interface.
One of the quick texts for SRB is the pipe cleaner test. Positive results are shown
in these examples.